1. Field of the Invention
This invention relates to a memory which employs a magnetic thin film.
2. Description of the Related Art
FIG. 1 is a diagram of a conventional magnetic thin film memory device disclosed in "Magnetic Thin Film Engineering" (p. 254, Magnetic Engineering Lecture 5; Maruzen Co., Ltd., 1977).
An example of how to manufacture the memory element will be discussed in the first place. A mask with rectangular holes is brought in tight contact with a smooth glass substrate G, onto which a vacuum deposited film of Fe, Ni about 2000.ANG. thick is formed within a vacuum apparatus. As a consequence, many magnetic thin film memory elements MF are manufactured in matrix at one time. A driving line to drive the magnetic thin film memory elements is obtained by photoetching copper strips on both surfaces of a thin epoxy resin plate or a thin polyester sheet in a manner that the strips on the one surface are orthogonal to those on the other surface. The lines on the both surfaces are rendered word lines and digit lines, respectively, and the memory device is assembled in a manner that each crossing point of the lines is arranged to overlap onto each memory element.
The principle of operation of the memory element will be depicted. The lines parallel to the easy magnetization axis in the drawing are word lines, while those orthogonal to the easy magnetization axis are digit lines. The digit line serves also as a sense line to read the storing state of information in the memory element.
In the figure, arrows A and B show the magnetization direction in the film in accordance with the storing state. Specifically, the upward arrow A in the drawing shows that information "0" is stored and the downward arrow shows that information "1" is stored in the memory element. Supposing that magnetic fields acting on the magnetic thin film by a digit current Id and a word current Iw are respectively Hd and Hw, when the current Iw of a unipolar pulse is allowed to run by selecting the word line W1, the magnetic field Hw acts to the whole of the memory elements MF below the word line W1, and the magnetization is directed on the axis of hard magnetization. At this time, pulse voltages of the opposite polarities are induced to each digit line which become reading voltages depending on whether magnetization is turned from the "1" state or "0" state. In recording, the digit current Id is fed so as to overlap the trailing edge of the Iw pulse, and in the condition of the magnetization being directed in the axis of hard magnetization, the magnetic field Hd of the polarity corresponding to an information signal is superimposed, thereby determining the magnetization direction in order to record information in the "1" state or "0" state. The value of Iw is set to generate the magnetic field Hw sufficient to turn the magnetization of the magnetic thin film from the easy magnetization axis to the axis of hard magnetization. The value of Id is set to generate the magnetic field Hd having about half the coercive force Hc of the magnetic thin film.
In conventional reading method, a minute electromagnetic induced voltage resulting from the rotation of the magnetization was used. Therefore, the S/N ratio at reading is so small that read-out was difficult. Moreover, since the electromagnetic induced voltage is proportional to the size of the magnetic moment, it is required to make the magnet thin film larger. In consequence, the magnetic field necessary for recording/reproducing is undesirably enlarged, thereby causing a hindrance to saving power. The amount of information stored per unit area is impossible to be increased.